Subdomains don't get authenticated with Letsencrypt

As long as I not create any subdomain Letsencrypt is working as expected.
When I create a subdomain and try to get a new certificate I get following error: urn:acme:error:unauthorized :: The client lacks sufficient authorization :: Invalid response (there is a 403 error only for the subdomain)
The authentication file is created but the url doesn't root/or cant access the file for the subdomain.
I created a testfile /user/local/ispconfig/interface/acme/.wellknown/acme-challenge/ test.txt

When I call the url from the main domain the file is found: http :// www. domain .tld/.well-known/acme-challenge/ test.txt
But when I call the url form the subdomain I get a 403 forbidden message.